// IO Monad 解决函子嵌套的问题  扁平化函子
const fs = require('fs');
const fp = require('lodash/fp');

class IO {
  static of(value) {
    return new IO(function () {
      return value;
    });
  }

  constructor(fn) {
    this._value = fn;
  }

  map(fn) {
    return new IO(fp.flowRight(fn, this._value));
  }

  join() {
    return this._value();
  }

  // 扁平化
  flatMap(fn) {
    return this.map(fn).join();
  }
}

let readFile = function (filename) {
  return new IO(function () {
    return fs.readFileSync(filename, 'utf-8');
  });
};

let print = function (x) {
  return new IO(function () {
    console.log(x);
    return x;
  });
};

let r = readFile('package.json')
  // .map(x => x.toUpperCase())
  .map(fp.toUpper)
  .flatMap(print)
  .join();

console.log(r);
